Considerable progress has occurred in the past few years towards understanding the critical variables that regulate T-cell growth. The T-cell growth factor, interleukin 2 (IL-2), has been identified as a single variably glycosylated polypeptide with Mr = 15,500, has been purified to homogeneity, and the gene encoding the peptide has been molecularly cloned. The mechanism of interaction of IL-2 with activated T cells has been found to be comprised of a classic, polypeptide hormone-receptor system. Monoclonal antibodies have been prepared to IL-2 and its receptor. Consequently, the cellular and molecular reagents are available for experiments directed toward understanding IL-2 receptor-mediated DNA duplication and mitosis. (HF)